This is used to pretty common, but because it is such a sensitive issue with high loads nowdays, most companies simply fire CSR's who get caught tweaking non-rev priorities. A few still do it, but most are afraid for their jobs.

If you suspect someone is doing this to you, and you are on your way to work tell them you will have to report it management as the reason you missed your show time. If they let someone on out of priority, they can't delete the evidence so they're going to get caught if anyone looks into it. I have seen managers meet an arrival airplane at the gate and hold deplaning until they get on board and physically verify who sitting in a certain seat.